FIG. 1 represents a basic diagram of a crystal oscillator. A crystal resonator 10 is connected between the input and the output of an inverter 12 formed by two complementary transistors. The output of the oscillator is taken at the output of the inverter.
The common mode of the inverter is fixed by a resistor 14 connected between the inverter's input and output. A frequency correction network comprising two capacitors 16 and 17, respectively connecting the resonator terminals to a reference voltage, enables adjustment of the oscillator's frequency.
FIG. 2 is a schematic plot representing the variations in time upon start-up of the oscillation frequency of an oscillator of the type of FIG. 1. From time t=0, the frequency increases progressively to asymptotically reach a nominal value.
As can be observed, the oscillator may not be immediately operational. Two conditions for use of the oscillator from start-up can often be distinguished. A time t1 defines conditions from which a microcontroller may be synchronized for certain start-up tasks not requiring great frequency accuracy. The frequency accuracy at time t1 is for example −1%. At a time t2, the oscillator should reach its maximum accuracy, for example 10 ppm.
A desire to use a microcontroller as of time t1 exists in the field of low-power applications where an apparatus regularly wakes up from a standby state to check whether there is a task to be performed. If there is no task to be performed, the apparatus switches back to standby state. It is therefore desirable for time t1 to be reached rapidly after start-up and for the power consumption necessary to reach this time to be minimum. It is desirable, for example, that time t1 be less than 800 μs.
Time t2, from which maximum accuracy of the oscillator is reached, is for example 4.5 ms. It may be desirable for the oscillator to consume a minimum power from this time on, in steady state, while guaranteeing the same accuracy.
It is however usual that the minimum current for keeping the oscillator in its steady state is insufficient for the oscillator to start-up fast enough to reach a time t1 within the times that are currently required (800 μs).
It is established that the oscillator start-up speed increases with the supply current. It is therefore useful to supply the oscillator with a high current at start-up for it to start quicker, and to then reduce the current in steady state to reduce power consumption.
Oscillator structures exist where inverter 12 is replaced by a variable gain current amplifier to regulate the gain inversely to the oscillation amplitude. The purpose of such a structure is mainly to help ensure that the oscillation amplitude in steady state does not clip, in order to prevent phenomena detrimental to the operation of the oscillator and the circuitry it supplies. In such a structure, the oscillator happens to be supplied with a higher current on start-up than in steady state.
Such an oscillator is described for example in U.S. Pat. No. 7,262,671. It comprises a transistor external to the inverter, connected in current mirror mode with the P transistor of the inverter, so that the inverter's transistor copies the external transistor's current. This external transistor has its drain terminal connected to the gate terminals of both the inverter's transistor and the external transistor. A current source biases the external transistor with a fixed current on which a regulation circuit superposes a variable current which is a reverse function of the oscillation amplitude.
The regulation has a linear range and is optimized to help prevent clipping in steady state. The characteristics of this regulation depend on the sizing of the transistors and are therefore likely to vary with temperature and with uncontrollable variations of the manufacturing process. Furthermore, the regulation is not intended for optimizing the start-up speed and the current consumption in steady state. On account of the fact that the circuit comprises branches with four transistors in series between the power supply terminals, it is not suitable for use under low supply voltages.